This invention is in the field of mechanically rotating choppers for electromagnetic radiation, and is particularly concerned with a chopper for use with a staring array of infrared detectors. In order to suppress background radiation in scanned infrared detecting systems, AC coupling is used. For staring arrays, however, some other method must be used. Most uncooled staring systems use a chopper that alternately provides a sharply focussed image and a blurred or scattered image as a background reference. Each detector of the array is thus alternately exposed to radiation from a specific point in the scene and radiation proportional to the local background radiation level. The algebraic difference between the alternate radiations (images) is the scene with background suppressed. The currently used choppers are made of a material transparent to infrared radiation (such as germanium) and are in the form of disks with open portion If the infrared scene is focussed by a fast objective lens onto the detector array through the open portions of the chopper, it will be out of focus when passing through the closed portions because of the optical thickness of the disk and the shallow depth of focus of the objective lens. While this type of chopper works well, it has disadvantages which the present invention overcomes. Specifically, the disk is expensive to make, and is very fragile.